There have been many investigations in recent years of the effects that small-geometry surface configurations can have on turbulent boundary layers. Particular attention has been paid to the provision of so-called riblet surfaces in which an array of small longitudinal ribs extend over the turbulent boundary layer region of a surface in the direction of fluid flow over the surface, and experimental results indicate that net or surface drag reductions of up to about 7% can be achieved.
In a paper "Drag Characteristics of V-Groove and Transverse Curvature Riblets" (presented by M. J. Walsh at the Symposium on Viscous Drag Reduction, Dallas, Tex., Nov. 7-8, 1979) reference is made to an early investigation (1966) by Liu, Kline, and Johnston in which drag reductions of 3-4% were obtained by reducing the turbulent bursting rate (i.e. the rate of break-up of the low speed longitudinal vortices or "streaks" that are characteristically formed in a turbulent boundary layer flow close to a wall) by employing rectangular fins. Walsh's paper reports the investigation of a number of alternative rib profiles and states that he was able to obtain a maximum drag reduction of 7% using V-groove riblets, this representing substantially the greatest reduction of skin friction drag that experimenters have been able to obtain up to the present.
The drag reduction may be associated with the ability of the riblets to limit random spanwise movements of the streaks, as has been suggested by a number of sources, e.g. R. E. Falco (AIAA-83-0377, AIAA 21st Aerospace Sciences Meeting, Jan. 10-13 1983, Reno, Nev. Johansen and Smith (AIAA-85-0547, AIAA-Shear Flow Control Conference, Mar. 12-14, 1985, Boulder, Colo.) have shown that cylindrical riblets of a smaller height than the V-groove riblets with which Walsh obtained has optimum results, have the effect of anchoring and/or creating sites of low-speed streaks in a limited region above the wall surface, but their experiments also showed drag increases of 3% to 8%. More recently, S. P. Wilkinson has attempted to follow Johansen and Smith by using blade riblets to anchor or create low-speed streaks by the use of riblets and then use V-groove riblets between the streak anchors to control bursting, but it is not yet reported whether a net drag reduction has been achieved thereby.
The results reported from these and other previous investigations all show the effect of riblets is rather limited, and that has led to the search for alternative solutions.
It has been proposed (U.S. patent application No. 686959 of M. J. Walsh et al - NASA Case LAR-13286-1) to reduce skin friction more effectively by providing a riblet surface with superimposed large-eddy break-up devices (LEBU's) as manipulators for the outer region of the boundary layer but such an arrangement adds considerable complexity and is susceptible to wear and especially accidental damage. In another proposal ("On the Drag Reduction of the Shark Skin", Bechert, Hoppe and Reif, AIAA-85-0546) it has been suggested that it may be appropriate to use so-called vortex generator surfaces which are configured so as to produce tangential mixing using local flow conditions over a ridge-pattern structure analogous to the configuration of scales on a shark's skin. It is claimed that this could lead to greater drag reduction, but it would also be inherently complicated and difficult to build up a simulation of the scaly surface that the investigators have concluded is responsible for the low drag characteristic.
In contrast to these attempts to reduce skin friction by rather complex means, riblet surfaces are less susceptible to damage and they can be formed relatively straightforwardly, e.g. by machining or pressing or by the application of a preformed, e.g. extruded, layer. It would clearly be preferable to employ such surfaces if they were able to give greater reductions of skin friction drag.
The mechanisms by which riblets influence the turbulent boundary layer over a wall is not yet fully understood and it is perhaps significant that different researchers have published conflicting data about the variation of net reduction of skin friction with rib length. If it can be postulated that they bring a turbulent flow closer to laminar flow conditions their efficiency is low, bearing in mind that in equivalent turbulent and laminar flow regions the skin friction drag with laminar flow would be 80% less than that with turbulent flow. If a more significant proportion of this potential improvement could be achieved, the use of riblet surfaces could offer marked advantages over the complex drag reduction systems described above.